The present invention relates generally to ceramic paper and methods of manufacture, and more specifically, to novel inorganic type ceramic papers having both high strength and good flexibility properties.
Ceramic paper generally refers to sheet products having thicknesses up to 1/4 inch made from and predominantly comprising ceramic fibers. Such papers find wide usage in smaller household appliances as heating element insulation, and also have many industrial uses, such as refractory applications.
In meeting specifications for such applications ceramic paper should be both flexible and strong. Desired minimum standards for tensile strength are at least 1000 gram load/inch with preferably 1500 gram load/inch as determined by TAPPI test method ASTM T404, and flexibility as measured by TAPPI folding endurance test method ASTM T423 which should provide a minimum of 150 fold cycles. In addition, ceramic papers should be resistant to thermal shock to avoid disintegration and flaking apart from heat.
Heretofore, ceramic papers relied principally on latex, polyvinyl acetate and phenolic resin type organic binders for achieving needed tensile strength properties. However, upon exposure to high temperatures these organic binders would frequently burn out causing outgasing and a loss in tensile strength. In an effort to overcome this problem, one alternative which had been tried was the production of an inorganic sheet without organic binders. However, virtually all attempts to prepare inorganic ceramic papers have so far proven unsuccessful in one way or another. Some attempts yielded very weak products, e.g. burn grade papers, while others resulted in formulating papers with limited flexibility. Hence, while substantial effort has been made to prepare papers with both high tensile strength and flexibility, heretofore retaining both properties in the same inorganic ceramic paper at commercially acceptable levels i.e. minimum flexibility of at least 1.75 percent elongation as measured by ASTM T423 and a tensile strength of at least 1000 grams/inch as measured by ASTM T404, has not been achieved.
Accordingly, it would be highly desirable to have inorganic ceramic papers which are both flexible and strong, and free of organic binders and the disadvantages associated therewith.